This invention relates to a method for the fabrication of thick film passive electrical components on diamond substrates for the microelectronics industry. More particularly, the method of this invention relates to depositing a thick film system of pastes onto diamond, and after firing, allowing the diamond to be incorporated into electronic packages using conventional joining techniques.
Integration of high power electronic devices into electronic systems typically requires the construction of an electronic package comprised of active and passive components. Since the manufacturing costs for microelectronic parts must be low, inexpensive fabrication methods are required. Many microelectronic components are manufactured by the methods of thick film technology, which typically utilize screen-printing techniques to deposit a thick film paste in a pattern to define the circuit element. The printing is followed by a high temperature treatment, i.e. firing operation, in a belt furnace, at up to about 900xc2x0 C., which sinters and stabilizes the paste components.
A passive circuit element consists of the passive material and a conductor material, which establishes its connection to the other elements of the circuit. Usually the conductor material is deposited and fired, followed next by the deposition and firing of the passive element material. An overcoat material may also be used to environmentally protect the passive material. In the case of a resistor element, for example, typically the conductor material is first printed and fired. The resistive material is then printed such that a portion of the printed resistive material is overlaid onto the terminating ends of the conductor, then fired. An overcoat, i.e. passivation layer, is printed over the resistive material and fired in a final step to environmentally protect the resistive material. Thus, the fabrication of a resistor element typically requires a series of three steps, each consisting of printing and firing of a thick film paste for the conductor, resistor, and overcoat. The sintering processes require a high temperature cycle, and each of the materials that comprise the parts of a resistor must, therefore, be compatible at these high temperatures during processing. A collection of compatible thick film pastes that are used in the fabrication of a particular circuit element is referred to as a xe2x80x9cthick film paste system.xe2x80x9d
The choice of the substrate material used for the mechanical support of the circuit elements is dependent upon the application. In high performance circuits used in telecommunications networks, the requirements for operation at high power and at high frequencies are increasing. In such a case, the substrates must have relatively high thermal conductivity and excellent dielectric properties. Synthetic diamond such as chemical vapor deposited diamond (xe2x80x9cCVD diamondxe2x80x9d) typically has a thermal conductivity that is many times greater than other conventional substrate materials currently used in the manufacture of thick film circuitry, such as aluminum oxide and beryllium oxide. Diamond also has a dielectric constant and loss tangent within the acceptable limits required for high frequency applications. Thus, diamond substrates have the potential to accommodate the demand for increasing power density and operational frequency requirements for high performance circuits.
High power devices, such as those used in high speed computing, microwave and RF telecommunications, typically use conventional ceramic materials such as beryllium oxide or aluminum nitride as substrates for thermal management. These materials are cut to shape from sheets and electronic circuits are fabricated onto the surfaces by the screen-printing methods discussed above. These conventional ceramic substrate materials are stable in the high-temperature oxygen-containing environment, e.g. air, required for sintering commercially available thick film pastes. Diamond, however, deteriorates at temperatures in excess of about 600xc2x0 C. in air. At typical temperatures and times required for complete sintering of prior art thick film materials in air, a diamond substrate is disintegrated.
There are a variety of thick film pastes commercially available that are formulated for sintering in a non-oxidizing ambient, e.g. nitrogen. These pastes are in the minority of commercially available thick film systems for manufacturing circuit elements. Similarly, there are resistor formulations and passivation overcoats that may be fired in nitrogen. These thick film pastes usually contain a glass constituent that does not reduce in a non-oxidizing atmosphere and becomes fluidic at about 900xc2x0 C. The glass wets to the substrate, resulting in a sintered microstructure that possesses robust adhesion and cohesion. Conductor and resistor thick film paste formulations are made by adding constituents that are more conductive than the glass, which lower the resistivity of the sintered structure to a desired specification. In these cases, the glass also wets to the additional constituent, retaining cohesion and adhesion. A commercially available thick film conductor paste based on a mixture of copper and glass powders, for example, has been formulated for firing in nitrogen at about 900xc2x0 C.
In addition to the limited selection of thick film pastes, the chemistry of the constituents are generally designed for bonding to oxide substrates, such as beryllium oxide. This particular material has dominated the microelectronics market for thermal management applications because of its relatively high thermal conductivity compared to aluminum oxide, and relatively low cost. Beryllium oxide, however, is considered to be an environmentally unsafe material and will be phased out of usage in the near future. Alumina is inexpensive, but has a relatively low thermal conductivity, and is not used for high-performance thermal management applications.
Recently, as the manufacturing cost of diamond has declined due to improved CVD diamond synthesis techniques, the demand for diamond components has increased. The need for CVD diamond in high power density applications is rapidly increasing as package sizes decrease and package power increases. Sheets of diamond substrates have recently become commercially available with the lateral dimensions and flatness that are amenable to the screen-printing techniques discussed above. As a result, a thick film paste system for fabricating microelectronic circuits on the surface of diamond substrates, diamond heat spreaders and components would be useful in many applications where enhanced thermal management is vital to the performance of microelectronic packages. Therefore, there is a commercial need for improved thick film paste materials and manufacturing processes for application of these paste materials to enable the use of diamond substrates for high-power, high frequency electronic circuit elements.
To-date, techniques for production of reliable thick film structures on diamond for microelectronic packages using commercially available thick film pastes have not been known. Generally, thick film formulations that work well with oxide-based substrates do not work well for diamond substrates. These difficulties are generally due to one or more of four factors:
(1) the requirement to sinter the pastes in an oxygen-containing environment which is incompatible with diamond,
(2) the lack of adhesion of available thick film formulations to diamond due to significant differences in the coefficient of thermal expansion (CTE) between diamond and constituents of most commercially available thick film pastes, which were originally developed for use on substrates with a significantly higher CTE that that of diamond, such as beryllium oxide or aluminum oxide,
(3) the lack of adhesion of available thick film formulations to diamond due to the lack of chemical interaction or bonding, between diamond and the constituents in most commercially available thick film pastes, which were developed for oxide substrates, and/or
(4) the lack of predictable resistance of conventional thick film paste materials (designed for oxide substrates) when applied to diamond substrates.
During attempts to formulate thick film resistor materials for diamond substrates, it was discovered that all commercially available thick film pastes that were tested failed to produce adequate adhesion, resistance, or temperature coefficient of resistance for use in commercial applications.
U.S. Pat. No. 4,639,391 and U.S. Pat. No. 4,985,377 teach barium or strontium borosilicate glass compositions and conductors in mixtures to obtain compositions exhibiting a wide range of resistances. There is no suggestion in either of these references of using diamond as a substrate material. Neither is there any suggestion of the entire thick film system (conductor, resistor, and passivation layers) that is required for the completion of a circuit element.
U.S. Pat. No. 5,631,046 refers to the metallization of diamond through the application of commercially available thick film pastes. A pre-treatment of the diamond in air at elevated temperature of 850xc2x0 C. for 10 minutes is required for good adhesion. Such a treatment results in the deterioration of the thermal conductivity of the diamond by forming cavities on the surface. The cavities form as a result of preferential attack by oxygen on grain boundaries, which are inherent in polycrystalline diamond synthesized by common growth processes. Thus, this treatment is not practical without diminishing, to a considerable extent, the quality of the thermal performance originally intended of the diamond.
Graphite resistors are fabricated by laser ablation of the diamond surface as mentioned in a publication, Weidner et al., xe2x80x9cLaser Induced Graphite Resistors in Synthetic Diamondxe2x80x9d, Int""l J. Microcircuits and Electronic Packaging, Feb. 2, 1996, p. 169. The temperature coefficient of resistance (TCR) for these resistors was not reported. This method of making resistors is inherently limited to low power applications. Since the resistor is formed from a graphite-like layer formed by laser machining, the resistive portion is essentially limited in thickness and the resistivity to narrow ranges, since the laser converts only a very thin amount of the diamond to a graphitic layer. The resistor, then, is limited to the electrical properties of graphite.
U.S. Pat. No. 5,853,888 refers to the surface modification of diamond to allow commercially available thick film pastes to adhere to the substrate. The modification consists of depositing thin films of chromium for adhesion, then aluminum oxide, followed by annealing in oxygen. Although this surface treatment results in a chemistry compatible with most commercially available thick film paste constituents, the additional cost of the surface treatment is high.
The invention relates to methods of applying (e.g., by painting or screen-printing) a thick film paste system onto the diamond surface to produce a diamond substrate having passive circuit elements. More specifically, the method allows for the fabrication of resistor elements having an sheet resistance of greater than about 1 ohm per square and a TCR within the range of about 0 to about xc2x1300 ppm/ xc2x0 C. (ohm per square is a unit of an electrical measurement of surface resistivity across any given square area of a material; it is the measurement of the opposition to the movement of electrons across an area of a material""s surface and normalized to a unit square, and is conventionally used in the art with respect to sheet resistance). Even more specifically, the method allows for the production of resistor elements having an sheet resistance of approximately 50 ohms per square, and a TCR between about xe2x88x92200 ppm/xc2x0 C. and +200 ppm/xc2x0 C.
The diamond substrate used in the product of this invention is preferably synthesized by chemical vapor deposition (CVD) methods. Typically, the diamond material is grown in flat sheets to a specified thickness, and then separated from the growth substrate. The diamond sheets may be polished to the required thickness and surface roughness while the opposing surfaces are kept parallel within the required specification or tolerances. Typical thickness of the diamond material is in the range of about 100 to 1000 micrometers, preferably about 300 to 500 micrometers. Typical average surface roughness of the diamond surface is 0.1 to 30 microns, depending on the frequency of the application. Laser machining can be performed, if necessary, prior to metallization in order to make via holes (xe2x80x9cviasxe2x80x9d) in the diamond or to define the diamond surface dimensions.
Electronic packages made using the present invention take advantage of the heat spreading, thermal conductivity, and electrical insulating properties of CVD diamond. The products of this invention have applications including thermal management of high power, high frequency circuits used in communication networks and other electronic circuits, and are particularly useful in microwave communications devices, where small components capable of dissipating relatively large quantities of heat are desirable.
In accordance with this invention, glasses made of barium, boron, and silicon (barium borosilicate) have been found to provide particularly good adhesion to CVD diamond after being sintered at temperatures of up to about 900xc2x0 C. In addition, it has been found that there is a class of resistor formulations based on essentially a barium borosilicate matrix that can be sintered in a non-oxidizing atmosphere.
In one embodiment, the invention relates to a method for producing circuit elements comprising:
(a) depositing onto a diamond or nitride substrate a patterned thick film resistive composition, said thick film composition comprising a borosilicate glass having one or more Group II metal oxides; and
(b) firing said thick film resistive composition in an inert environment at a temperature and for a dwell time sufficient to form a resistive layer.
In another embodiment, the invention relates to methods that, in addition to the above steps, include
(c) depositing a patterned layer of a thick film conductor paste onto at least a portion of the substrate; and
(d) firing said thick film conductor paste in an inert environment at a temperature and for a dwell time sufficient to result in a patterned conducting thick film having a resistance of no greater than about 30 milliohms per square.
The conductive layer can be deposited onto the diamond substrate prior to deposition of the thick film resistive composition, or subsequently, so long as at least a portion of the layers overlap. A passivating dielectric layer can be deposited over the other layers and fired to protect the completed circuit element.
In another embodiment, the invention relates to a circuit element which comprises:
(a) a diamond or nitride substrate;
(b) a patterned conducting thick film layer; and
(c) a patterned resistive thick film layer comprising a borosilicate glass containing one or more Group II metal oxides.
If desired, the resistive thick film layer can also contain quantities of conductive material to vary the sheet resistance of the layer. The conductive material can include metals, such as nickel, chromium, vanadium, zirconium, iron, hafnium, niobium, tantalum, tungsten, molybdenum, titanium, and mixtures and alloys thereof. In addition, if desired the resistive thick film layer can include quantities of materials capable of modifying the thermal coefficient of resistance. These can include semiconducting materials. Suitable TCR modifiers are those materials that can cause the TCR of the composition to vary, in particular those materials that have a negative TCR. TCR modifiers can include silicon, germanium, carbon, boron, semiconducting compounds, oxides, carbides, nitrides, ruthenates, and combinations thereof.